Polyurethane coatings are known to have the advantages of high gloss, chemical resistance and abrasion resistance. When formulated properly they can display flexibility, impact resistance and toughness. Additionally, when aliphatic or cycloaliphatic multifunctional isocyanate monomers are utilized in the composition, they exhibit outstanding weatherability and lightfastness characteristics.
Epoxy thermosetting resin coatings are known, the thermosetting property being achieved by a crosslinking cure. Epoxy coatings have the advantages on cure of good adhesion, dielectric or insulation properties, dimensional stability, fatigue resistance and low shrinkage, among others.
It has been proposed to combine the two, using polyepoxides with 1,2-oxirane functionality as a crosslinking agent for a urethane oligomer that has terminal primary amine functionality and has been dispersed in water with the aid of a neutralizing, volatile organic acid such as acetic acid. The epoxy as a crosslinker is named and disclosed as such in WATERBORNE POLYMERIC FILMS by A. Tortorello, M. Kinsella and R. Gearon, technical report AFWAL-TR-80-4197, Wright-Patterson Air Force Base, Dayton, Ohio, 1980. Neutralization was one hundred percent, since nothing less than the equivalent amount of acetic acid is mentioned. This is confirmed by Tortorello U.S. Pat. No. 4,427,804 (1984), where "an equivalent amount" of acetic acid is specified and by U.S. Pat. No. 4,489,179 (1984) where it is stated "Neutralization of the amine content by the acid should be at least 50% of stoichiometry . . . 100% being preferred."
An important function of the ketimine containing component of the Tortorello patents is that it, along with the volatile neutralizing acid, is used as an emulsifying agent for liquid and/or solid polyepoxides. The level of neutralization of the amine terminated component of the present invention by volatile organic acid ranges from 25-40% of the stoichiometric level. The partly neutralized amine terminated component of our system, in the form of a colloidal dispersion, does not act as an emulsifying agent for liquid or solid polyepoxide resins, therefore, both the specified neutralization range of our system, and the process of combining components in our method are significantly different from the Tortorello teachings.
It is noteworthy that the physical properties and water resistance of the various systems utilizing ketimine blockage of primary amine functionality as described in WATERBORNE POLYMERIC FILMS, above, were well below the level that could be expected from high performance solvent borne urethane coatings. We have found the neutralization level of 25-40% is critical for the development of optimum physical properties during cure: as neutralization is increased above 40%, physical properties of films cured at room temperature are significantly reduced, and are not comparable with existing solvent containing urethane coatings products. The systems of this invention, on the other hand, have physical properties and water resistance properties similar to those of high quality solvent borne moisture cure and two part urethane coatings systems.
Both patents above are limited by the requirement that the ketone of choice to form a ketimine blockage of primary amine functionality is ". . . a ketone which is hindered to prevent reaction with a secondary amino hydrogen atom, this hindered ketone being selected from the group consisting of non-cyclic ketones containing one group selected from methyl or ethyl and a second group containing at least four carbon atoms, and cyclic ketones which are substituted in the alpha position."
Using ketimine formation techniques to be described, ketimines made from hindered non-cyclic ketones were compared extensively with ketimines using non-hindered, non-cyclic ketones to terminate isocyanate functional prepolymers. After dispersion and blending with an emulsion of polyepoxides, we repeatedly found properties of the cured coatings to be identical. Therefore, the use of the "hindered" ketone is not necessary and is of no benefit in this invention. A non-hindered ketone is our ketone of choice because it can be more readily and effectively stripped from the dispersion, thus eliminating substantially all of the undesireable volatile content.
The coating systems described in the examples and claims of the above prior art patents are further limited as they contain substantial amounts of volatile solvents. U.S. Pat. No. 4,489,179 states that the epoxy crosslinker "requires the presence of organic solvent . . . [enough] to substantially completely dissolve the polyepoxide . . . [and] 60% and 80% solids content solutions are particularly effective." The room temperature curing system as described would have a volatile organic content (VOC) substantially in excess of the Environmental Protection Agency (EPA) limits.
As will be explained below, only minimal quantities of volatile organic solvents are needed in either of the two components of this invention, to the degree that the VOC falls well below current EPA requirements.
The viscosity of the liquid polyepoxide of U.S. Pat. No. 4,427,804 (e.g. 12,000 cps) and of the 60-80% non-volatile solution of the polyepoxide of U.S. Pat. No. 4,489,179 is such that great shearing forces, and high shear mixing equipment, are required to create the emulsion of resin in the ketimine blocked component. The resultant emulsion is "stable in the sense that no significant change is observed for at least one hour". To one versed in epoxy technology, this statement carries the implication that the pot life of the composite emulsion would coincide with the period of emulsion stability and it would be expected the the pot life would not exceed more than a few hours.
Again our invention differs in that both components are in shelf stable (up to one year or more) emulsion and colloidal dispersion form, and are combined just prior to application by simple mixing techniques. The pot life of the composite is typically measured in terms of days, rather than hours.
An important feature of this invention is the use of a substantial equivalency excess of the amine terminated urethane oligomer to epoxide equivalency, as opposed to the stoichiometric ratio suggested in the Tortorello document and patents. As will be explained, this equivalency excess changes the curing mechanism in such a way as to promote the development of high performance urethane film characteristics.